Conventional lead-acid batteries, such as those used for automobiles and trucks, generally include a number of cells disposed in a battery housing. Each cell typically includes a plurality of positive and negative battery plates or electrodes, and separators are sandwiched between the plates to prevent shorting and undesirable electron flow during the reactions which take place during manufacture and use of the batteries. The plates and separators are immersed in electrolyte disposed in the cells, the most common type of electrolyte being aqueous sulfuric acid and infrequently the electrolyte is in the form of a gel. The positive plate generally is constructed of a lead-alloy grid covered with lead oxide, while the negative plate generally contains lead as the active material, again covering a lead alloy grid.
In most battery constructions the battery housing includes a box-like base to contain the cells and which is made from a moldable resin. The housing is generally rectangular in horizontal cross-section, the cells being provided by vertical casing, the cover including terminal bushings and a series of filler holes to allow electrolyte to be added to the cells and to permit whatever servicing is required. To prevent undesirable spillage of electrolyte from the fill holes, most prior art batteries have included some sort of filler hole cap.
The electromotive potential of each battery cell is determined by the chemical composition of the electroactive substrates employed for the electrochemical reactions. For lead-acid batteries, such as those described above, the potential is usually about two volts per cell, regardless of cell volume. Vehicles manufactured by original equipment manufacturers (OEMs) typically require twelve volt batteries, so most of today's batteries include six cells (6 cells.times.2 volts per cell=12 volts). The size of the housing for the battery is selected for the "envelope" for a particular vehicle, i.e. the physical dimensions defined by the vehicle manufacturer for containment of the battery in the engine compartment.
Battery electrolyte spillage or spewing can be caused by a number of factors, including vibration or tilting as a vehicle maneuvers during normal use. Electrolyte escape may also be caused by battery overheating, a problem especially pronounced in recent years with smaller engines, which tend to run hotter than prior engines.
In addition to preventing spillage or spewing of electrolyte from the cells, the battery cover design and the filler caps need to perform an important and different function. This is because gases are liberated from lead-acid batteries during the charge and discharge reactions. Such reactions start at the time the battery is originally charged (called the "formation process"0) by the manufacturer or by the retailer or vehicle manufacturer. They also occur during normal operation of the battery. Factors such as high current charge and discharge conditions, and changes in temperature, can affect the rate at which gas evolution occurs. Control of gas generation and evolution in lead-acid battery construction is particularly important, because the gases are hydrogen and oxygen, and it is important to vent such gases in a controlled way from the battery to prevent pressure buildups in the housing which could lead to electrolyte leaks, housing failures, or most significantly explosions within the housing. It is also desirable, and well known, to prevent an external flame from entering the battery through gas exhaust ports.
The control for releasing gases from the electrolyte cells has often been accomplished by providing gas release slots on the primary cover in addition to the barrel openings. One drawback for providing such gas release slots is that during the formation process, the gas release slot become an additional passageway through which electrolytes escape. As a result, during the formation process, these gas release slots need to be blocked off in addition to the barrel openings.
Two of the problems previously mentioned, i.e. electrolyte spewing and gas evolution, are really interrelated and important in the construction of an effective cover and vent system. For example, electrolyte may enter the vent cap through several mechanisms. One mechanism is through vibrational or tilting flow of electrolyte in the cap, and another is through a mechanism frequently referred to as pumping. The latter occurs when gas evolved in the battery bubbles from the cells and carriers or forces electrolyte out the fill hole in into the cap. When electrolyte enters the caps of some prior designs it may be carried out the exhaust passageway and cause damage to external battery components, such as the battery terminals or adjacent engine components.
One particularly useful venting system to minimize the entrance of electrolyte into the flow path of the evolved gases is described in commonly owned U.S. Pat. No. 5,702,841. The venting system includes a base containing the cells covered by a primary cover having a barrel extending into each cell. A secondary cover having baffles and splash guards is bonded to the primary cover to form chambers which allow the gases to escape while inhibiting the electrolyte from following the gas flow path. This particular system, however, requires filling the cells with electrolyte prior to bonding the secondary cover to the primary cover. The step of bonding the secondary cover to the primary cover, such as by heat sealing, after the cells are filled with the electrolyte adds an additional expense to the battery assembly process. It is preferable to accomplish all of the bonding processes prior to introducing the electrolyte into the cells.